ABSTRACT Dystroglycanopathies are neuromuscular diseases that result in progressive muscle wasting, which decreases quality of life and often leads to early death. Dystroglycanopathies result from mutations in genes that encode proteins that participate in dystroglycan (DG) glycosylation. DG is a transmembrane receptor for extracellular matrix (ECM) proteins and its glycosylation is necessary for cells to adhere to their surrounding ECM at both neuromuscular junctions (NMJs) and myotendinous junctions (MTJs). The contribution of disrupted cell-ECM adhesion to altered structure and function of the neuromuscular system in the context of dystroglycanopathies is poorly understood. This is partially because dystroglycanopathies caused by the same genetic mutation have variable clinical presentation, including severe congenital onset muscular dystrophy with eye/brain involvement, congenital myasthenic syndrome, and milder adult-onset limb girdle muscular dystrophies. There are multiple roadblocks to understanding the phenotypic variation of these incurable diseases. The neuromuscular system involves coordinated development of neural and muscle tissues to form NMJs, but mechanisms are not fully understood. The effects of disrupted primary motor neuron development on subsequently developing secondary motor neurons, muscle, and NMJ structure and function are not understood in the context of muscular dystrophies such as the dystroglycanopathies. A genetic model of dystroglycanopathies in a vertebrate model that allows longitudinal studies of neuromuscular development is needed to address these gaps. We generated a zebrafish model of gmppb-associated dystroglycanopathy. Our preliminary data suggest that gmppb is required for primary motoneuron, NMJ, and muscle development and/or homeostasis. Our central hypothesis is that gmppb is required for normal motor axon pathfinding; and that early disruption in motor axon pathfinding leads to defects in neuromuscular structure and homeostasis. We will test this hypothesis by conducting longitudinal studies that will test whether/how primary motoneuron development impacts muscle homeostasis. Elucidating cellular mechanisms is a crucial first step to understanding the molecular mechanisms of phenotypic variation in development and disease. This research is innovative because our preliminary data are the first to show early motoneuron axon pathfinding defects. No longitudinal studies of neuromuscular development in vertebrate models of dystroglycanopathies have been conducted. This study will have a significant impact on our understanding of roles for protein glycosylation in neuromuscular development. Thus, completion of this grant will provide new insight into how initial motor axon development affects neuromuscular development and homeostasis. This information is a critical foundation for understanding the basic biology underlying abnormal neuromuscular phenotypes in the dystroglycanopathies. Understanding these basic mechanisms is an important first step towards identifying future therapeutic targets. Taken together, this grant will significantly impact the field of neuromuscular development and homeostasis.